This invention relates to an improved method of producing a lubricating oil viscosity index improver and dispersant and the product of this improved process. In another aspect, this invention relates to a lubricating oil comprising the product of the improved process.
Lubricating oil viscosity index improvers which have dispersant properties are known in the art, as taught by Kiovsky in U.S. Pat. No. 4,141,847. Kiovsky's viscosity index improver is prepared by reacting a selectively hydrogenated star-shaped polymer having at least four arms of polymers or copolymers of diolefins and monoalkenyl arenes with alpha-beta unsaturated carboxylic acid or acid derivative, and then reacting the resulting intermediate with an amine.
The process incorporated by Kiovsky to produce his viscosity index improver includes reacting the selectively hydrogenated star-shaped polymer with an alpha-beta unsaturated carboxylic acid or acid derivative at a temperature of between about 150.degree. C. and about 300.degree. C. for one to twenty hours, in a solvent containing 1% to 10% by weight polymer. The product of this functionalization reaction is then reacted with a C.sub.1 to C.sub.18 amine containing one to eight nitrogen atoms. Although the product of Kiovsky's process offered a significant advance in the art of lubricating oil additives, it has the disadvantage of requiring that large volumes of materials be held at high temperatures for extended reaction times. Separation of the product from the solvent is also required. This causes the operating costs for Kiovsky's process to be relatively high.
Attempts have been made to accomplish the functionalization of the base polymer in a more economical fashion by reacting the selectively hydrogenated polymer with the functional groups in an extruder. An example of such an attempt is Hayashi et al. U.S. Pat. No. 4,670,173. Hayashi reacts a hydrogenated block copolymer with an unsaturated carboxylic acid in the presence of a free radical initiator. The reaction is performed in either a solvent-free system or in a solution. The reaction may be carried out in any suitable vessel, device or apparatus, but is desirably performed in the absence of a solvent in an extruder, a Banbury, a two-roll mill, or the like. The product of the functionalization process is then reacted with a primary amine-containing compound in either a solvent, or in a solvent free system.
Hayashi also teaches that an optional and important aspect of his invention is that the blending device imparts high mechanical energy to such an extent that sufficient force is applied to the copolymer chains to cause scission or breakage thereof. This scission is said to be useful when the block copolymer is of higher molecular weight than that which is desired. As opposed to being an advantage, Applicant has found this scission to be a disadvantage. Further, applicant has found that in addition to scission, imparting high mechanical energy causes coupling and crosslinking of the base polymer causing a certain portion of the resultant functionalized polymer to have a significantly higher molecular weight than the initial polymer. Because of coupling, crosslinking and scission, the functionalized polymer produced by the reaction of the polymers and the functionalization agent upon imparting high mechanical energy has a broad molecular weight distribution. The rate at which polymers degrade while in service as a lubricating oil viscosity index improver is a strong function of the polymer's molecular weight with high molecular weight polymers degrading much more rapidly than lower molecular weight polymers. The "thickening effect" is also a strong function of molecular weight with high molecular weight polymers having a much greater effect on composition viscosity than low molecular weight polymers. A polymeric viscosity index improver with a broad molecular weight distribution will therefore lose thickening effect more rapidly as the polymer ages in service than a viscosity index improver with a narrow molecular weight distribution. It is therefore very desirable to have a viscosity index improver which has a narrow molecular weight range.
An advantage of Hayashi's process is that it may be carried out in a conventional extruder at a relatively high throughput when compared to solution functionalization processes. A process having this advantage, with a reduced level of scission, crosslinking and coupling is needed for the production of lubricating oil viscosity index improvers-dispersants.
It is therefore an objective of this invention to provide a process to produce a lubricating oil viscosity index improver-dispersant in which the resultant viscosity index improver-dispersant has a reduced level of scission, crosslinking and coupling.